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    Re: Sextants in space
    From: Frank Reed
    Date: 2018 May 22, 10:43 -0700

    Paul Dolkas, you wrote:
    "It seems they are thinking ahead to using them for backup devices in the Orion spacecraft, which (like Apollo) is being designed for lunar & interplanetary flights. It's important to remember that in such missions, being able to rendezvous with another spacecraft isn't a luxury, - it is literally your only ticket home."

    Employing a sextant in rendezvous is an interesting idea (that's how Buzz Aldrin used the sextant in 1966 --52 years ago), except for one thing: there are exactly zero mission-critical rendezvous scenarios funded or even in serous planning. The only rendezvous even contemplated for the Orion spacecraft is docking with the proposed "Lunar Gateway" which is a proposed small space station in lunar orbit, and it's not mission-critical. If the docking fails, they go home. There is no scenario where a sextant would be a critical tool.  And yet there's a more important issue. There's no way a handheld sextant could be used effectively from inside the Orion spacecraft (more below).

    The caption from the PAO says:
    "Onboard ISS, crew members test a hand-held sextant for emergency navigation meant for use on Orion Exploration Mission 2. The team selected the ISS Cupola module as the experimental location, as it provides a good analog for window and crew placement in future exploration vehicles such as Orion."

    Notice how the PAO has spun this, intentionally or not. They have captioned the image in the present tense suggesting that the astronaut photographed here is already testing the sextant. Yet, beyond the slightest doubt, this is a photo in a simulator on the ground. Notice also that the caption specifically suggests that his sextant might be used on "Orion Exploration Mission 2" which has neither any significant rendezvous component nor any requirement for autonomous navigation. Notice also that the Cupola module is described as a "good analog for window and crew placement". This is where the whole thing goes off the rails. Nothing about the scene in the Cupola would resemble the interior of the Orion capsule. Like the Apollo spacecraft, the windows on the Orion spacecraft are not intended for optical observations, and they are small. The angular field of view would be less than 45° even with a sextant pressed right against a window, and the windows are not optically flat. There is no way that a manual sextant could serve more than a trivial function in such an environment. By contrast, Skylab had a huge optically-flat viewing window, and the Cupola aboard the ISS is an amazing viewing platform, a wonderful luxury for the astronauts. Of course, in twenty years, there might be some other spacecraft with big windows, but it won't be Orion...

    You added:
    "I think one of the reasons they are repeating these experiments is that at NASA, once a program (e.g. Gemini, Apollo, or Skylab) is done with, the government typically lays off most of the workforce, and the people and all their experience get scattered to the winds. Then a few years go by, a new program starts up, and a whole new generation comes along and re-invents the wheel."

    Yes, it's wasteful, isn't it? There is a real thing called "institutional memory". Aspects of collective culture like safety and risk management generally involve human judgment in a way that is difficult to transmit by paper (or other recorded media). You need the experience. But playing with sextants? Do we lose the science of sextant navigation every time NASA turns out the lights at night? No. This is not at all like institutional memory. We know exactly how sextants work. We know in detail how the Gemini astronauts used their manual sextants. They filed reports. We know in detail how the Apollo astronauts used their sophisticated semi-automated sextants. And we also know what navigational experiments were performed aboard Skylab, and we know that they were redundant and pointless activities. Absolutely nothing of lasting value was learned in the Skylab manual navigation experiments (these were mostly lunar distance observations, by the way, so I have a lot of sympathy for them despite this). And yet here we go again! The failure in instituional memory here is not "how do we use this-here funny sextant thang up in outer space?" The failure in institutional memory is being unable to recognize pointless, wasteful, make-work experiments. It's a failure in judgment.

    You added:
    "The people who had done the research had written their reports, and retired or been laid off decades ago.  So it’s a bit sad but not surprising that we are doing it again."

    It's more than sad. It's a bad joke. Those reports did not retire. The reports were not laid off... This is just the sort of make-work activity that space stations thrive on. It looks like science. It looks like space exploration. It convinces Congress to continue paying for re-supply missions.

    You suggested:
    "As for the use of digital cameras or other such devices, I can only speculate that those approaches presume that some level of functionality in the electronics remains after (multiple) system failures."

    If all of your electronics fail in space, you're dead. The idea that you would be saved because you have a manual sextant is a ridiculous delusion. The backup you want in case of disaster has been well-understood since the early days of Apollo: you want a backup spacecraft with an independent electrical and computer system. And you want "mission control" handling your guidance and navigation and managing whatever problems arise. Those were the critical lifesaving backups on Apollo 13. The backup celestial navigation methods were of no use to them at all. They were even told, "don't bother" when they asked Houston if they should take some sights to align the inertial platform. And yes, we all know about the burns where they used visual celestial alignments to maintain orientation. This was a long, long way from the use of a sextant.

    You added:
    "Since your life literally depends on some fairly precise navigation, it would be reassuring to know that if everything craps out you still have a manual backup that won’t."

    Fairly precise navigation indeed. And standard off-the-shelf star trackers have accuracies at least 100 and likely 10,000 times better than manual sextant sights. Recently, we were talking about using lunar distance sights for position-fixing on Earth using the same sort of sights they experimented with aboard Skylab. You get a fix accurate to about six nautical miles assuming decent high-accuracy observations (0.1 minutes of arc after averaging). This positional error is directly proportional to the linear distance from the Moon or planetary body from which angles are being measured:
      Positional error (n.m.) = [angular error (m.o.a.)]·[(distance from planet center)/(radius of earth)]
    So if I am four million miles from Mars, in some distant future Mars mission, that implies I can fix my position in space using a manual sextant to within about 100 nautical miles, if I measure angles with high accuracy, to the nearest tenth of a minute of arc. Unfortunately, this is not good enough by a wide margin, and that's only four million miles, not far at all by interplanetary standards. And I must reiterate: there are no such missions planned except in the PAO art studio. Experiments on emergency navigation decades before any mission might fly are exercises in public relations... and wishful thinking.

    You concluded:
    "Kinda like knowing how to do a sight reduction if your calculator falls overboard."

    So... I need paper navigation at sea using tables because my calculator might fall overboard? But my paper tables could never fall overboard?? There's no logic here. Put your solar-powered calculator in your sextant case. Check it once a year --mostly to make sure it hasn't "walked off". Apart from obsolete licensing requirements, paper tables are only for devoted hobbyists, antiquarians and apocalyptic survivalists (in case the change of focus confused anyone following along, I'm talking now about ocean navigation --not space navigation). Carrying that analogy back to space navigation, no, I am not advocating $10 calculators for space flight. Independently-powered, handheld supercomputers with gigabytes of fixed and working memory with high-quality, built-in sensor suites should be available to the crew of any deep-space exploration spacecraft. The only question is whether they would be Android or iPhone... and can you get a cell signal on the Moon?

    Frank Reed

       
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